1. Field of the Invention
The invention relates generally to an electrical circuit for detecting malfunctions of an electrical device, and methods of constructing and utilizing same. More particularly, the present invention pertains to an electrical circuit for detecting when an electrical device loses current, and for providing an indication or alarm of such malfunction.
2. Description of the Relevant Art
In the art it is often desirable to know when electrical devices have malfunctioned so that the defective devices can be repaired or replaced. For this purpose, it is known to use indicators in cooperation with the electrical devices to indicate when the devices have malfunctioned, whereby maintenance personnel can quickly diagnose and fix electrical problems.
In this regard, known indicators have been constructed which receive a current when the monitored electrical device has malfunctioned, which current is then used to power an indicator light or the like. As an example, one of applicant's prior inventions, sold under the trademark CAL-LITE, is such an indicator light. The CAL-LITE device has been used in cooperation with power capacitor assemblies by being placed in parallel with the fuses associated with the capacitor assemblies. In the event a capacitor assembly fails it would short-circuit, thereby blowing the fuse or fuses connected thereto, and thereby also providing a driving current to the CAL-LITE indicator or indicators.
However, a new generation of electrical devices, called "metallized", have emerged in recent years which do not short-circuit upon failure. As an example, a conventional capacitor includes two metallic foil layers (or plates) separated by a film (polypropolene) layer and a sheet of paper insulation, whereas a metallized capacitor simply includes a film or paper substrate having thin metal coatings deposited on opposite sides thereof. In the conventional capacitor a short-circuit develops across the two metallic layers if there is a breakdown in the paper insulation, and this short circuit blows the fuse(s) associated with the capacitor. On the other hand, the metal coatings in the metallized capacitor are very thin, whereby they simply vaporize in a small area if a fault develops through the substrate. Thus, once the metallized coatings vaporize in the metallized capacitor, the current path will be gone, and the fault will be extinguished. This phenomenon is known as "self-clearing". It is common to all metallized capacitors, wet, "damp" or dry.
The momentary fault current that occur in a metallized capacitors is too small to constitute a short-circuit by fuse standards. Thus, previous indicators, such as CAL-LITE, cannot be used to indicate when a metallized capacitor has experienced such momentary faults because the fuses have not been blown and, correspondingly, no power is supplied to the indicators.
As a protection for metallized capacitors, pressure-actuated interrupters, as shown at 8 in FIGS. 1-3, are provided therewith. If the capacitor experiences many self-clearings within a short span of time, the self-clearings develop enough gas pressure to make the capacitor cell bulge. This causes the pressure-actuated interrupter 8 to open, as shown in FIG. 3, which takes the capacitor cell off line. This is a normal end-of-life sequence for a metallized capacitor assembly, and results in an open circuit. Again, however, conventional indicators such as CAL-LITE cannot be used to indicate that the capacitor has gone off line because there is an open circuit, whereby no current is supplied to the indicator to actuate it.
Similar problems exist in relation to other electrical devices.
Hence, there is a need in the art for an indicator which can indicate when electrical devices, such as metallized-type electrical devices, have malfunctioned by detecting a loss or reduction of current in the electrical devices.